Liquid crystal display elements have been used in various devices such as measuring equipment, automotive panel, word processor, electronic note, printer, computer, TV, clock, advertisement panel board, and etc., including watch and electronic calculator. The representative of the liquid crystal display method includes TN (twisted nematic) type, STN (super twisted nematic) type, VA (vertical alignment) type using a TFT (thin film transistor), and IPS (in-plane switching) type. The liquid crystal composition used in the liquid crystal display element is demanded to be stable to external factors such as water, air, heat, and light. It is also demanded to exhibit a liquid crystalline phase in as wide temperature range as possible including the room temperature, as well as it is demanded to have a low viscosity and require lower drive voltage. Furthermore, the liquid crystal composition comprises several to dozens of compounds in order to obtain the most appropriate values of dielectric constant anisotropy (Δ∈) or/and refractive index anisotropy (Δn) depending on individual display element.
A vertical orientation type display uses a liquid crystal composition whose Δ∈ is negative, which is widely used in e.g., liquid crystal TVs. On the other hand, lower voltage drive, higher response, and wider operating temperature are demanded in all drive types. Namely, it is demanded to have a positive range of Δ∈ with a larger absolute value, having a lower viscosity (η), and higher transition temperature (Tni) between nematic phase and isotropic liquid phase. Also, by regulating the product (Δn×d) of the Δn value and a cell gap (d), it is required to adjust the Δn value of the liquid crystal composition in a suitable range in accordance with the cell gap. In addition, when the liquid crystal display element is applied to TVs, high-speed response is weighed heavily, and therefore, the liquid crystal composition is required to have a small rotational viscosity (γ1).
Conventionally, in order to compose a liquid crystal composition having a low value of γ1, it was usual to use a compound having a dialkylbicyclohexane frame (see Patent Reference No. 1). However, although a bicyclohexane compound exhibits a high effect to reduce the value of γ1, generally, it tends to have a high vapor pressure, and especially, such a tendency can be remarkably found in a compound with a short alkyl chain length. In addition, Tni tends to be low. Therefore, as an alkylbicyclohexane compound, it is often to use a compound having a total side chain length with a carbon atom number of 7, and enough examinations have not been made for a compound having a shorter side chain length.
A liquid crystal composition was known which uses a dialkylbicyclohexane compound having a short side chain length (see Patent Reference No. 2). This composition balances the physical properties as a whole by using a compound having three ring structures as a negative dielectric constant anisotropy compound as much as possible, using a compound having a difluoroethylene frame. With respect to the difluoroethylene frame used in this composition, however, there is a problem as to its low stability with respect to the light, and therefore, it is demanded to develop a liquid crystal composition without using such a compound.
On the other hand, as the application of the liquid crystal display elements has come to spread, there have been large changes in their use and manufacturing method. In order to cope with the changes, it has been demanded to optimize the characteristics more than the basic physical properties as conventionally known. Namely, as the liquid crystal display element using the liquid crystal composition, a VA (vertical alignment) type and an IPS (in-plane switching) type have been widely used, and in addition, a display element having a super large size such as size 50 or more has become available and used. With upsizing the substrate size, as for the injection method to substrate of the liquid crystal composition, the dropping method (ODF=One Drop Fill) has been mainly adopted from conventional vacuum injection method (see Patent Reference 3). Along with this change, a problem comes to the surface when dropping a liquid crystal composition on the substrate, leaving a drop trace and causing deterioration of the display quality. Here, the drop trace is defined as a phenomenon when a trace made by dropping a liquid crystal composition appears white even in a black display.
For the purpose of high-speed response of the pretilt angle control of the liquid crystal material in the liquid crystal display element, there have been developing a PS liquid crystal display element (polymer stabilized; polymer stabilization), and a PSA liquid crystal display element (polymer sustained alignment; polymer maintenance orientation) (see Patent Reference No. 4), and therefore, the issue of the problems has been more significant. Usually, these display elements are characterized in that a monomer is added in the liquid crystal composition, and the monomer in the composition is hardened. On the other hand, a liquid crystal composition for active matrix is required to maintain a high voltage retention rate, so that a compound having an ester linkage is restricted from usage, and therefore, there are few kinds of compounds available.
As a monomer used for the PSA liquid crystal display element, an acrylate system is mainly used, and such an acrylate compound generally has an ester linkage. Acrylate compounds are usually not used as a liquid crystal compound for active matrix (see Patent Reference 4). When a large quantity of an acrylate compound is included in a liquid crystal composition for active matrix, the generation of drop trace is induced, thereby increasing the problem of the aggravation of the yield of the liquid crystal display element due to the display defectiveness. In addition, aggravation of the yield can also become problematic when the additives such as antioxidant and light absorption agent are added in the liquid crystal composition.
As a method to control the drop traces, disclosed is a method to polymerize a polymerizable compound mixed in the liquid crystal composition to form a polymer layer in the liquid crystalline layer in order to control the generating drop trace in relation with the orientational control film (Patent Reference No. 5). However, in this method, there is a problem in which there occurs ghosting in the display which is caused by to the polymerizable compound added in the liquid crystal composition, and therefore, the effect to suppress the drop trace is insufficient. Therefore, it has been demanded to develop a liquid crystal display element in which basic characteristics as liquid crystal display elements are maintained while it is unlikely to cause the ghosting and the drop trace.